1. Field of the Invention
This invention relates to the field of galvanizing wire such as steel wire to provide corrosion resistant wire having a bright, silvery luster and, more particularly, relates to the high speed, in-line, continuous production of galvanized, low carbon steel wire that can be process annealed without destroying the corrosion resistant coating or its silvery luster to provide a more ductile, galvanized wire of improved corrosion resistance.
2. The Prior Art
There have been numerous attempts at galvanizing steel wire or other elongate steel forms followed by drawing the wire down to a smaller diameter and many patents describing such attempts have been granted over the past century and more. The prior art processes were limited to the production of those types of galvanized wires having high hardness, low formability, and brittleness due to the inability of such prior art processes to provide for annealing after drawing without loss of some or all of the zinc coating, thereby reducing or destroying corrosion resistance, and/or oxidizing the surface of the zinc coating thereby destroying most or all of its appealing, bright, silvery luster.
U.S. Pat. No. 3,730,758 discloses a process for galvanizing steel strips by flash coating the steel strip with a metal, e.g., zinc, by vacuum deposition or electroplating followed by hot-dipping to apply a second coating of zinc. It states that the first flash coating can be of aluminum since an aluminum addition is normally made to galvanizing baths in order to decrease interface alloy formation of the zinc with the surface of the strip. It fails, however, to disclose a drawing or forming step after galvanizing, to provide the advantage of lower speed required through the galvanizing step and a faster speed through annealing, or to provide any annealing step to relieve the stresses introduced by the drawing or forming step or to disclose the provision of means by which the galvanized strip could be annealed without impairing the bright, silvery luster of the zinc coating.
Each of the U.S. Pat. Nos. 936,637; 1,133,628 and 1,816,617 discloses the precoating of steel wire with zinc by electrolytic means or with zinc flue dust followed by hot-dip galvanizing. None of these patents, however, teach the drawing down of the galvanized wire or the annealing of same after drawing and to do so would result in a rough unsightly surfaced wire having lower corrosion resistance, because the zinc would be oxidized and partially vaporized during the anneal.
U.S. Pat. Nos. 101,264; 2,286,073; 2,288,762 and 2,482,978 each disclose methods of galvanizing wire followed by drawing to elongate the wire and reduce its thickness. None of these patents disclose, teach or suggest the use of an annealing step after drawing to relieve the strains created by drawing, and to reduce the hardness and increase the formability of the drawn galvanized wire. If annealing were performed after drawing, the wire would be expected to have an unsightly rough dark finish due to heavy oxidation.
U.S. Pat. No. 2,378,458 discloses a process for precoating steel wire with copper or retard the formation of zinc-iron alloy upon galvanizing and to render the zinc coating more ductile and more easily worked. There is no disclosure, teaching or suggestion of annealing after drawing the galvanized wire to relieve the stresses created by the drawing operation or that a bright, silvery finish on the wire would result.
U.S. Pat. Nos. 2,152,842 and 2,326,629 disclose the coating of steel billets with a paint containing 70 parts Al, 23 parts sal ammoniac and 7 parts zinc to protect the billets from surface deterioration during subsequent reheating to hot rolling temperatures. The paint forms an alloy with the steel billet during the hot forming operation. Subsequent working and alternation of the alloy with the steel billet are required to give the alloy a degree of pliability and, after hot rolling, the billet may be cleaned and alloyed with zinc. After this complicated processing to form a suitable alloy on the steel billet surface, a second metal, e.g., aluminum or zinc is united to the surface in strip form or by congealing progressively or other involved alloying procedures utilizing aluminum and zinc powders can be used. In spite of this complicated procedure, the workpiece still is susceptible of separation of the coating from the steel billet and special dies are necessary to prevent separation.
U.S. Pat. Nos. 213,015; 2,268,617; 2,359,095 and 2,472,393 deal with various procedures utilizing copper coatings but none disclose, teach or suggest annealing after drawing nor do they disclose, teach or suggest that a bright, silvery finish is obtainable after annealing to relieve the strains of drawing.
The text titled Galvanizing (Hot-Dip) by Heinz Bablik, 3rd Edition, published by E. and F. N. Spon, Ltd. in 1950 discusses in depth the use of aluminum additions to hot-dip galvanizing baths. For the purpose of retaining the brightness of the zinc coating, amounts of 0.02% in the bath are adequate. Higher amounts of the order of 0.2 to 0.3% result in structural changes in the coating by delaying the reaction between the solid iron base and the molten zinc so that at short dipping times and low dipping temperatures no reaction takes place at all (p. 208). At higher concentrations of aluminum it is impossible to make liquid zinc react with solid iron and higher temperatures and longer dipping times are needed (p. 209). It suggests that pregalvanizing in molten zinc (without aluminum) produces a thin iron-zinc alloy with which the zinc bath containing aluminum (0.4%) can react readily but if higher concentrations of aluminum are added to the bath not even pregalvanizing is able to cause the reaction to start (p. 213). With a zinc bath containing 5 % aluminum not even hot dip pregalvanizing is able to cause the initiation of the reaction at 440.degree. C. (824.degree. F.) (p. 214). At 0.24% aluminum (no pregalvanizing) the inhibition of the reaction is so strong that at 440.degree. C. (824.degree. F.) and one hour's immersion in the bath no reaction has yet occurred (p. 221; see also Metals Handbook Vol. 2, Heat Treating, Cleaning and Finishing, p. 500, published by American Society for Metals in 1964).
In order to obtain good adhesivity, according to the Bablik text, there must be a reaction between the iron base and the zinc coating and if inhibited by the presence of aluminum then the iron zinc alloy does not form and adhesivity suffers (p. 306-307). As little as 0.01% aluminum is able to impede the formation of dross almost completely (p. 377). The addition of aluminum to galvanizing baths is little practiced but if it is decided upon then 0.1 to 0.3% is added; however, due to the absence of iron-zinc alloy layers the resulting zinc coatings are quite flexible but are so light and thin as to be generally not desirable (p. 462). When galvanized wire is annealed it turns dark grey or green, partially vaporizes, and becomes rough surfaced due to oxidation and the iron-zinc reaction continues during annealing to increase brittleness and adhesivity (p. 463). Aluminum additions also substantially increase the fluidity of the bath and make the production of uniform heavy coatings difficult (p. 265, 266; see also The Making, Shaping and Treating of Steel, published by United States Steel, J. M. Camp et al, 6th Edition, 1951, p. 942).
None of these literature references, namely, Bablik, Metals Handbook or Camp, et al teach or suggest any advantage in using relatively large amounts of aluminum, namely, amounts sufficient to provide in the galvanizing bath a eutectic alloy having a melting point below that of zinc (e.g. about 420.degree. C.) but not less than the melting point of the eutectic alloy of zinc and aluminum containing 5 wt.% aluminum. None of these references disclose, subsequent to galvanizing, the drawing and annealing of the galvanized wire without substantial removal or embrittlement of the zinc coating, loss of corrosion resistance or loss of brightness by oxidation or roughening of the coating surface. The references teach away from the use of the relatively large amounts of aluminum because of the purported adverse effects of the iron-zinc alloying reaction needed in a small degree to provide adhesivity and because of the purported undesirable increase of fluidity of the zinc bath with increasing aluminum content causing the bath to drain off the steel being coated and leaving coatings that are too thin. None of these references teach the sequence of steps of galvanizing, drawing and annealing.
The Galvanizing Handbook by John R. Daesen, pp. 17-106, published by Rheinhold Publishing Corporation in 1946; the text Alloyed Zinc Coatings, Paul E. Schnedler, p. 2, published by Armco Steel Corp. in a paper given at the "Proceedings of the Galvanized Committee", Sept. 1970; Fe-Zn Alloy Formation During Galvannealing, by Smith et al, J. of the Iron and Steel Institute, December 1972, pp. 897, 899; and Bablik, supra, at pages 462 and 463 each discuss the "galvannealing" process in which the zinc coated steel is subjected to elevated temperature, e.g., 850.degree. F. to 1200.degree. F. or as high as 1450.degree. F. directly after removal from the hot zinc bath in order to promote the iron-zinc alloying reaction and thereby improve adherence of the coating to the steel base. The resulting galvannealed wire has an unappealing grey surface due to oxidation of the surface zinc layer and roughness due to cracking of the coating and is unsuitable for drawing because of the increased brittleness imparted by growth of the iron-zinc alloy layers.
The addition of the low amounts of aluminum, 0.08 to 0.25%, is recommended by the Daesen reference, the Schnedler reference and the Smith et al reference to inhibit the formation of the brittle alloy phases while permitting the development of the more workable iron-zinc alloy. Smith et al, p. 599 states that aluminum levels higher than 0.15% result in major discontinuities in the overall kinetics of alloying. Daesen, P. 60 states that the sacrificial nature of the protection given by zinc to iron requires for most work a thicker coat than can be secured uniformly in a bath of high (0.15%) aluminum content and recommends the maintenance of the aluminum content at a figure low enough to promote practical fluidity, 0.05% or thereabouts. The Schnedler reference confirms that, according to AISI terminology, the word "galvanneal" is usually applied to alloyed coatings with a light commercial coating weight. None of these references recommend the addition of relatively large amounts as mentioned above, of aluminum in the galvanizing bath nor do they disclose, teach or suggest the sequence of galvanizing, drawing and annealing.